3-D EM Inversion to the Limit
In a typical day of mineral exploration, an airborne electromagnetic (EM) system may cover 500 km of line, recording data at 10-m spacings. Wide-band data at the 50 000 stations then is interpreted to pick areas worthy of follow-up. In Australia, where conductive regolith is common, there may be 500 local anomalies in this data set. The first task, therefore, is to classify these anomalies by geometry and conductivity. Reduction of the original EM data at each station to only two parameters—the estimated free-space inductive and resistive limits—greatly reduces the complexity of the interpretation problem. The reduction involves stripping and correcting for background host effects. The inductive limit is a function of target geometry alone, whereas the resistive limit is linearly related to target conductivity for an isolated body, but also includes geometric effects. The calculation of forward and inverse 3-D conductivity models using these simple limits is possible in seconds on a fast personal computer. This allows for automated and interactive classification of the sources of anomalies. The results also can provide starting models for more careful inversion or modeling. The major limitation at present is that the method does not model current gathering, which takes place when conductors are in contact.